Ethers and alcohols are high octane components which contribute significantly to the quality of motor gasoline. The introduction of ethers and alcohols into motor gasoline as part of gasoline reformation generally increases the amount of oxygen in the gasoline. This additional oxygen combined with restrictions on aromatic hydrocarbons and heavy metals in finished gasoline is expected to result in the reduction of ozone-forming volatile organic compounds, exhaust nitrogen oxide emissions, and toxic emissions from motor vehicle exhaust.
High octane ethers for motor gasoline production are generally produced by a combination of an isoolefin with a monohydroxy alcohol such as methanol or ethanol in an etherification process. The etherification process can also be used as a means to produce pure isoolefin by cracking of the product ether. For instance, pure isobutylene can be obtained for the manufacture of polyisobutylenes and tert-butyl-phenol by cracking methyl tertiary butyl ether (MTBE). The production of MTBE has emerged as a predominant etherification process which uses C.sub.4 isoolefin as the feedstock. A detailed description of processes, including catalyst processing conditions, and product recovery, for the production of MTBE from isobutylene and methanol are provided in U.S. Pat. Nos. 2,720,547 and 4,219,678 and in an article at page 35 of the Jun. 25, 1979, edition of Chemical and Engineering News. The preferred process is described in a paper presented at The American Institute of Chemical Engineers, 851h National Meeting on Jun. 4-8, 1978, by F. Obenaus et al. The above references are herein incorporated by reference. Other etherification processes of current interest are the production of tertiary amyl methyl ether (TAME) by reacting C.sub.5 isoolefin with methanol, and the production of ethyl tertiary butyl ether (ETBE) by reacting C.sub.4 isoolefins with ethanol.
The problem with producing ethers from isoolefin is that the feedstock is usually derived from a petroleum or natural gas stream which must first be converted into an isoparaffin, followed by the alehydrogenation of the isoparaffin to an isoolefin, and finally, the etherification of the isoolefin with alcohol. Processes are sought which provide more direct lower cost routes to such ethers.
Alternatives to petroleum based technologies are sought as a route to high octane blending components for motor gasoline and reformulated gasoline having an increased content of oxygenates. A number of approaches to producing higher branched oxygenales such as C.sub.4.sup.+ alcohols and aldehydes have been attempted by converting a synthesis gas comprising a carbon oxide and hydrogen in a simple step process. These processes have generally been characterized by severe operating conditions, low conversion, and low catalyst selectivity to the branched oxygenate product. An example of this approach is found in EPO patent application 0,335,092. European Patent Application 0,335,092A2 to W. Falter et al. discloses a method of producing alcohol mixtures with an increased portion of isobutanol directly from synthesis gas such as CO and H.sub.2, or CO.sub.2 and H.sub.2, or mixtures of CO.sub.2 and CO with H.sub.2 or other gases containing CO, CO.sub.2 and H.sub.2 over a catalyst comprising a base of zirconium, zinc, and manganese oxides and up to 10 wt % of a base compound. The base compound may include an alkali and/or an alkaline-earth metal and/or ammonia. In addition, the catalyst may contain from 0.01 to 2 wt % palladium in elemental or compound form. Other metals such as Au, Ag, Cu, Sc, Y, Lanthanides, Ru, Rh, Os, Ir, and Pt are also disclosed as elements which can be contained in the catalyst. The reaction takes place at temperatures of 420.degree.-825.degree. C. and pressures between 10 and 480 bar. The catalyst is prepared by co-precipitation or successive precipitation of the corresponding metal salt solutions such as nitrates. In a related article entitled, "Isobutanol from Synthesis Gas," published in Catalysis Letters, Vol. 3 (1989), pages 59-63, the same inventors disclose an active and selective Zr--Zn--Mn--Li--Pd catalyst for a one-step synthesis of isobutanol from synthesis gas. They suggest that the use of palladium increases the selectivity to isobutanol by favoring methanol synthesis and suppressing methane formation. They particularly point out that at pressures less than 10 MPa, the isobutanol selectivity decreases, favoring methane formation. They also point out the critical nature of the temperature influence on isobutanol. At 645.degree. K., the isobutanol yield is 4%, but at 715.degree. K., the isobutanol yield increases to 45%. At higher temperatures the isobutanol yield decreases.
Other approaches to the production of higher branched oxygenates have focused on a two-step process wherein the first stage is the conversion of synthesis gas by well-known methods to methanol and the second step is the conversion of methanol to higher branched alcohols. Examples of such approaches are characterized by the vapor phase conversion of a mixture of methanol and ethanol over a solid catalyst employing a condensation reaction. Generally, these approaches have not provided the catalyst activity and selectivity levels necessary to offer a commercially viable route from methanol to the higher branched oxygenates. In addition, none of the approaches have demonstrated the ability to provide higher catalyst activity for the conversion of methanol in the absence of ethanol to produce such higher branched oxygenates as isobutanol.
U.S. Pat. No. 5,095,156 to Radlowski et al. relates to a continuous vapor phase condensation process to convert a C.sub.2 or higher alcohol, and one or more C.sub.1 or higher alcohols to a mixture containing at least one higher molecular weight alcohol such as butanol over a catalyst which is essentially magnesium oxide. The magnesium oxide component may, in addition, be supported by titania, alumina, silica, boria, zirconia, and a carbonaceous material such as charcoal. The isobutanol produced by the condensation reaction is separated, dehydrated, and reacted with additional methanol to form MTBE. In a related work by Wataru Ueda, Tetsuo Kawabara, Talmo Ohshida and Yutaka Morikawa, published in the Japanese Chemical Society, Chemical Communications, on pages 1558-1559 (1990), disclosed a synthetic method for the production of higher alcohols from methanol over a magnesium oxide catalyst at atmospheric pressure and elevated temperature. Ueda indicated that magnesium oxide showed the best catalytic activity in the reaction of methanol and ethanol selectively yielding propan-1-ol and 2 methylpropan-1-ol, zinc oxide catalyzed the alehydrogenation of ethanol to ethanol and zirconium oxide catalyzed the dehydration of alcohols to ethers. Magnesium oxide prepared by precipitation from magnesium nitrate was shown to exhibit poor activity and poor selectivity to higher alcohols.
In a paper entitled, "Synthesis of C.sub.2.sup.+ Oxygenales from Methanol at Atmospheric Pressure over Alkali-promoted Zinc-Chromium Oxide Catalysts, " by Luca Lietti et al., which appeared in Applied Catalysis, Volume 70, pages 73-86, in 1991, zinc-chromium oxide based catalysts were disclosed for the synthesis of higher oxygenates from methanol and hydrogen. Lietti et al. found that potassium-promoted zinc-chromium oxide, while decomposing a large part of methanol to carbon monoxide and hydrogen, also produces C.sub.2.sup.+ oxygenates. Lietti et al. concluded that alkali addition plays a crucial role in the formation of C.sub.2.sup.+ oxygenates over the zinc-chromium oxide catalyst.
An article by B. Mahipal Reddy et al., entitled "A Single-Step Synthesis of Isobutyraldehyde from Methanol and Ethanol over CuO--ZnO--Al.sub.2 O.sub.3 Catalyst," published in the Journal of the Chemical Society, Chemical Communication, pages 997-998, in 1992, discloses a catalyst for the production of isobutyraldehyde and its derivatives such as isobutanol over a CuO--ZnO--Al.sub.2 O.sub.3 catalyst from mixtures of methanol and water. In a further development, Reddy et al. in an article entitled, "Synthesis of Isobutyraldehyde from Methanol and Ethanol Over Mixed Oxide Supported Vanadium Oxide Catalysts," published in Applied Catalysis A: General, volume 96, pages L1-L5, in 1993, discloses the use of mixed oxides including TiO.sub.2 --Al.sub.2 O.sub.3, TiO.sub.2 --SiO.sub.2, TiO2--ZrO: and TiO.sub.2 --SiO.sub.2 --ZrO.sub.2 wherein the V.sub.2 O.sub.5 /TiO.sub.2 --SiO.sub.2 catalyst showed the better total conversion and product selectivity.
An article by Fey-long Wang et al. entitled, "Catalytic Synthesis of Isobutyraldehyde from Methanol and n-Propyl Alcohol over Titanium Oxide--Supported Vanadium Oxide Catalysts, published in Industrial Engineering Chemistry Research, volume 32, pages 30-34, in 1993, disclosed a process for the synthesis of isobutyraldehyde which is a raw material for producing isobutyl alcohol. Fey-long Wang et al. selectively produce isobutyraldehyde from methanol and ethanol in one step by using titanium oxide-supported vanadium oxide as a catalyst.
U.S. Pat. No. 2,971,033 to Martin W. Farrar disclosed a process for the manufacture of higher molecular weight alcohols from alcohols of lower molecular weight by carrying out the reaction in the presence of potassium carbonate, magnesium oxide, and copper chromite. The reaction was characterized by low conversions.
U.S. Pat. No. 3,972,952 to Roger T. Clark discloses a solid catalyst composition for the vapor phase conversion of methanol and ethanol to higher linear alcohols, particularly n-propanol, over a catalyst comprising 85-97% alumina and 2-14% of a base promotor selected from the group of oxides, hydroxides, and basic salts of alkali and alkaline earth metals with between 0.1 and 1 percent of a platinum group metal such as ruthenium, rhodium, palladium, osmium, iridium, and platinum. The process was carried out at a temperature range of about 200.degree. C. to 400.degree. C. and a pressure between about 6.7 MPa to about 33 MPa (1000 and 5000 psig) and space velocity of about 2000 to about 10,000 hr.sup.-1, but produced very small amounts of isobutanol.
U.S. Pat. No. 4,533,775 to Joseph R. Fox et al. discloses a process for the upgrading of lower alcohols to higher molecular weight alcohols by contacting the lower alcohol with a reaction promotor having a composition including a metal acetylide and a methyl hydride, and mixtures thereof.
Thus, the conversion of syngas to isobutanol, which can be readily dehydrated to isobutene, has received a significant amount of interest in the past several years. The major effort has focussed on the direct conversion of syngas to isobutanol, generally using catalysts based on alkali-modified methanol synthesis catalysts. Typically, the productivities of these catalysts are low (&lt;100 g isobutanol/kg catalyst/hr) and they co-produce methanol in amounts exceeding the stoichiometric requirement for the production of MTBE. In contrast, conventional methanol synthesis produced methanol with &gt;99% selectivity and productivities of &gt;1000 g methanol/kg catalyst/hr. The present invention has focussed on the development of a new process that can be used to convert lower alcohols to isobutanol. Processes are sought for the conversion of lower alcohols to higher, branched oxygenates such as isobutanol which provide the activity and selectivity of a commercially viable process.